1. Field of the Invention
The present invention generally relates to wireless base station equipment and, more particularly, to wireless base station equipment in a mobile communication system.
Recently, efforts have been undertaken to develop communication methods for a mobile communication system which use frequencies efficiently. The growing importance is being attached to the Code Division Multiple Access (CDMA) technology because it can achieve high-capacity communication.
2. Description of the Related Art
FIG. 1 is a block diagram showing a conventional CDMA wireless base station equipment. Referring to FIG. 1, data to be transmitted is encoded by an encoder 10 using spread spectrum technique and is converted into an analog signal by a D/A converter 12. A modulator 14 modulates the analog signal using, for example, quadrature modulation technique. The modulated signal is amplified by a variable-gain amplifier 16 and is turned into a high-frequency signal by a frequency converting circuit 18. The high-frequency signal is amplified by a high-power amplifier (HPA) 20 and is supplied to an antenna 26 via a duplexer 22 and a coaxial cable 24.
The high-frequency signal received via the antenna 26 is supplied to a low-noise amplifier (LNA) 20 via the coaxial cable 24 and the duplexer 22. The signal amplified by the low-noise amplifier 20 is turned into an IF signal by a frequency converting circuit 32. The IF signal is supplied to a demodulator 36 via an AGC amplifier 34 for quadrature demodulation. The demodulated signal is converted into a digital signal by an A/D converter 38. A decoder 40 decodes the digital signal using the spread spectrum technique and outputs the decoded signal.
For example, Japanese Laid-Open Patent Application No. 4-322521 teaches varying the transmission power of a wireless base station depending on the number of used channels. More specifically, the base station drops the transmission power thereof when the ratio between used channels and available channels exceeds a predetermined level, so as to reduce a cell size. As a result of the drop, mobile stations outside the size-reduced cell become incapable of receiving a signal from the base station and are forced to communicate with another base station. In this way, the number of used channels is prevented from increasing.
When the transmission power of a base station drops, the surrounding mobile stations receive less signal power. That is, the cell size is reduced in terms of coverage of the base station. However, no change results in the reception of the base station. Even after the reduction in the transmission power, the base station continue to receive spurious radiation from the mobile stations outside the size-reduced cell of the base station. In the CDMA system, if the power of the spurious radiation exceeds a predetermined level, a bandlimited signal is lost in the spurious radiation, resulting in a failure for the base station to discern a desired signal from the spurious radiation. Channel capacity in the CDMA system is determined by a ratio between the power of received spectrum-spread signals and the power of a decoded bandlimited signal. Unless there is a reduction in the power of spurious radiation, the benefit of increased channel capacity is not produced.
One conceivable way to control the spurious reception level is to vary the tilt angle of an antenna provided in a base station in accordance with the number of used channels. However, if the transmission power is fixed, the transmission power at a boundary of a cell remains relatively high. Since a mobile station chooses a host base station based on the level of electric field intensity of a control signal received from a plurality of base stations, the conventional system has a disadvantage in that it cannot ensure that a mobile station located at a boundary of a cell cannot be smoothly handed off to another base station.